The present invention relates to temperature control systems for integrated circuit chips (IC-chips). More particularly, the present invention relates to temperature control systems which circulate a fluid thru heat exchangers that are coupled to the IC-chips such that the temperature of the IC-chips stays within a few degrees of a selectable set point while the IC-chips undergo large step increases and large step decreases in power dissipation as they are tested.
After an IC-chip is initially fabricated, it must be tested in order to determine if all of the circuitry which is in the IC-chip operates properly. This testing is often done via a xe2x80x9cburn-inxe2x80x9d test wherein the IC-chip is kept above its normal operating temperature while it is sent a series of test signals. Such a burn-in test is performed because it greatly shortens the time period during which certain types of failures will occur within the IC-chip, if those failures are going to occur at all.
In the prior art, the burn-in test usually is performed by the steps ofxe2x80x941) placing multiple IC-chips in sockets on several printed circuit boards, 2) moving the printed circuit boards that are holding the IC-chips into an enclosed tester which has a heater, and 3), blowing hot air from the heater with fans such that the hot air flows across the IC-chips while they are sent the test signals. Such an enclosed tester, with its heater and fans, is relatively inexpensive; however, it has several major limitations.
For example, as the number of transistors within a single IC-chip increases, the maximum amount of electrical power which the IC-chip dissipates also increases. Thus, a point is eventually reached where the maximum variation in power dissipation of multiple IC-chips on several printed circuit boards is simply too large to be regulated by convection with air.
Also, it sometimes is desirable to sequentially test different subsets of the IC-chips which are held on the printed circuit boards; rather than test all of the IC-chips at the same time. But when the number of IC-chips that are being tested changes from a small subset to a large subset, then a large step increase will occur in their total power dissipation. This step increase occurs because the IC-chips that are being sent the test signals dissipate a much larger amount of power than the IC-chips that are not being sent the test signals. Similarly, when the number of IC-chips that are being tested changes from a large subset to a small subset, then a large step decrease in their total power dissipation will occur.
The above step increase and step decrease in power dissipation presents a particularly difficult problem because while the testing occurs, the temperature of the IC-chips needs to be precisely maintained within just a few degrees of a set point temperature. However, when power dissipation of the IC-chips takes a step up, the amount of heat which must be removed from the IC-chips in order to keep their temperature constant increases rapidly. Likewise, when the power dissipation of the IC-chips takes a step down, the amount of heat which must be added to the IC-chips in order to keep their temperature constant decreases rapidly.
Currently in the integrated circuit industry, there is a need for a temperature control system which can maintain the temperature of multiple IC-chips within a few degrees of a set point temperature while their total power dissipation undergoes step increases and step decreases of over twenty-kilowatts. Accordingly, a primary object of the present invention is to provide such a system.
In accordance with the present invention, a system for maintaining the temperature of IC-chips near a set point, while the IC-chips undergo large step increases and large step decreases in power dissipation as they are tested, has the following structure:
1) a hot fluid circuit in which a hot fluid circulates from a reservoir through heat exchangers and back to the reservoir, and in which the heat exchangers exchange heat by conduction between the hot fluid and the IC-chips;
2) a sensor which generates a temperature signal that indicates the temperature of the fluid at a particular point in the hot fluid circuit;
3) an electric heater which operates with a fast response to add heat to the fluid returning to the reservoir as a function of the temperature signal;
4) an analog valve which operates with a slow response to add a cold fluid to the reservoir as a function of the temperature signal; and where,
5) the slow response of the analog valve is compensated for by varying the amount of heat from the heater while the analog valve concurrently adds cold fluid to the reservoir.
In one embodiment, the electric heater adds heat to the fluid returning to the reservoir in response to a signal SHOT(n) which equals K1eOUT1 plus K2eIN plus K3d(eIN)/dt plus K4∫eIN, and the analog valve adds the cold fluid to the reservoir in response to a signal SCOLD(n) which equals K5eOUT2 plus K6eIN plus K7d(eIN)/dt plus K8eIN. Here, the terms eIN, eOUT1 and eOUT2 vary with the sensed temperature, and K1 thru K8 are constants. These signals for SHOT(n) and SCOLD(n) cause an operating mode where heat is added by the heater, and simultaneously, cold fluid is added through the analog valve.
One feature of the above simultaneous mode of operation is that the net temperature of all of the fluid which is entering the reservoir can be rapidly reduced. This rapid cooling cannot be obtained by the analog valve alone because the valve is a mechanical component whose speed of operation is inherently limited by mechanical inertia.
To achieve the rapid cooling, the heating power of the heater is reduced quickly via the output signal SHOT(n) while the flow of cold fluid through the valve slowly increases or even stays constant. Typically, the electric heater operates at least ten times faster than the analog valve.
The above rapid cooling effect is used when the IC-chips that are being tested undergo a large step increase in power dissipation. Due to that rapid cooling effect, the temperature of the fluid which enters the heat exchangers is kept within xc2x11xc2x0 C. of the set point temperature, even when the total power dissipation of the IC-chips that are being tested steps up by over twenty kilowatts.